With the spread of optical communication, the communication speed through a metropolitan optical communication network connecting a relay station has been increasing from 10 Gbit/s to 25 Gbit/s and further to 40 Gbit/s. This metropolitan optical communication network requires transmission across a long distance of 40 to 80 km with a single-mode fiber (SMF) for 10 Gbit/s, for example, and its important issue has been reduction of the size, power consumption, and chirping of optical transmission modules. Meanwhile, the required transmission distance normally decreases in reverse proportion to the square of the bitrate (modulation rate).
Generally, external modulation methods, which involve only small chirping, have been used to perform high-speed and long-distance transmission as above. Among them, electro absorption (EA) modulators utilizing the electro absorption effect have superior characteristics for reduction of the size and power consumption, integratability with semiconductor lasers, and so on. In particular, an integrated semiconductor optical element (EA-DFB laser) including an EA modulation element and a distributed feedback (DFB) laser, which has good single-wavelength characteristics, monolithically integrated on a single semiconductor substrate has been widely used as a light emitting device for high-speed and long-distance transmission. For the signal light wavelength, a 1.5 μm band, within which the propagation loss of the optical fiber is small, or a 1.3 μm band, within which the chirping is small, is mainly used.
Generally, in optical communication, the optical signal is required to be maintained at constant light intensity. In conventional practices, part of the optical signal is split, its light intensity is monitored, and the electric current to be injected into the DFB laser is controlled so as to maintain the monitored light intensity at a constant level. This is referred to as APC (automatic power control).
FIG. 1 illustrates a conventional light intensity monitoring method for performing the APC. In FIG. 1, a DC drive current is applied to a DFB laser part, and a bias voltage and an RF (signal) voltage are applied to an EA modulator part through a bias-T. As a result, a light beam from the DFB laser part is modulated by the EA modulator part and output as a modulated light output. The output light beam is converted into a parallel light beam by a lens 115, converged by a lens 117, and then input into an optical fiber 118.
Here, changes in light intensity can be monitored by a light detector 120 by splitting part of the parallel light beam with a mirror 119. Then, feedback is applied so as to increase the drive current to the DFB laser part if the light intensity drops whereas feedback is applied so as to decrease the drive current to the DFB laser part if the light intensity rises. In this way, the APC is possible.
Meanwhile, in FIG. 1, the mirror 119, serving as a splitter, is provided at a position at which the modulated light output is yet to be input into the optical fiber. Alternatively, part of the modulated light output after being input into the optical fiber can be split by an optical coupler and monitored.
Next, another conventional configuration will be described. As one of the standards for building next-generation ultrahigh-speed networks, 100 Gigabit Ethernet (registered trademark) (100 GbE) is under development (see Non Patent Literature 1). In particular, 100 GBASE-LR4 and 100 GBASE-ER4 are considered promising, which involve data exchange between buildings separated by a middle to long distance (up to 10 km) and between buildings separated by a very long distance (up to 40 km). In the above standard, an LAN-WDM method is used in which 25 Gb/s (or 28 Gb/s) data are set for each of four predetermined optical wavelengths (e.g., four wavelengths of 1294.53 to 1296.59 nm, 1299.02 to 1301.09 nm, 1303.54 to 1305.63 nm, and 1308.09 to 1310.19 nm) and multiplexed to generate a 100 Gb/s signal.
The LAN-WDM uses a wavelength multiplexing optical transmitter module. For the wavelength multiplexing optical transmitter module, it is important to reduce its size, energy consumption, and chirping. An integrated semiconductor optical element (EA-DFB laser) has been widely used which uses an external modulation method, in which the chirping is small, and includes an EA modulation element and a DFB laser monolithically integrated on a single semiconductor substrate.
FIG. 2 illustrates the configuration of a conventional wavelength multiplexing optical transmitter module used in 100 GbE. An optical transmitter module 323 including a wavelength multiplexing optical transmitter, which is a single semiconductor chip 322, as a light source, outputs a multiplexed modulated signal light beam into an optical fiber 321. The semiconductor chip 322 includes four DFB laser parts 301 to 304, four electro absorption (EA) optical modulator parts 305 to 308, and a single multi-mode interference 4×1 optical multiplexer 313. In other words, the semiconductor chip 322 includes four EA-DFBs in which the DFB laser parts 301 to 304 and the EA modulator parts 305 to 308 are connected to each other respectively to thereby integrate the DFB lasers and EA modulator parts. Also, input waveguides 309 to 312 and an output waveguide 314 are connected to the MMI 4×1 optical multiplexer 313.
Each of the DFB laser parts 301 to 304 outputs a continuous light beam, and the laser oscillation wavelength bands of the DFB laser parts 301 to 304 are 1294.53 to 1296.59 nm, 1299.02 to 1301.09 nm, 1303.54 to 1305.63 nm, and 1308.09 to 1310.19 nm, respectively. Note that the above four wavelength bands are usually referred to as lane 0, lane 1, lane 2, and lane 3 from the shortest wavelength side, respectively.
The EA optical modulator parts 305 to 308 include absorption layers of the same composition and, in accordance with inputs being individual RF signals (at 25 Gb/s or 28 Gb/s), convert the continuous light beams from the DFB laser parts 301 to 304 into 25-Gb/s or 28-Gb/s modulated signal light beams. The modulated signal light beams output from the EA optical modulator parts 305 to 308 are output into the waveguides 309 to 312, respectively.
The MMI optical multiplexer 313 multiplexes the four modulated signal light beams, which differ in wavelength, and outputs them as a single bundle of wavelength-multiplexed light beams into the output waveguide 314. The single bundle of wavelength-multiplexed light beams is emitted into a space as a scattered light beam 315, changed into a parallel light beam 317 by a lens 316, passes through an isolator 318, converged into a converged light beam 320 by a second lens 319, and coupled to a fiber 321.
Meanwhile, though not illustrated, besides the above, the optical transmitter module 323 includes a temperature sensor (e.g., thermistor) for the semiconductor chip 322, a Peltier element for temperature control, and DC power sources for supplying power to the DFB laser parts 301 to 304 and the EA optical modulator parts 305 to 308. The optical transmitter module 323 also includes a modulator driver and radio-frequency line termination resistors for driving the EA optical modulator parts 305 to 308, and signal lines and a control circuit for controlling the amplitude, bias voltage, electric cross point of the modulator driver. Further, an electric signal waveform shaping circuit and a clock extraction circuit as well as a circuit that reduces the influence of variation in power supply voltage may be provided before the modulator driver in some cases.
As the EA optical modulator parts 305 to 308, InGaAlAs-based tensile strained quantum wells are used, which have a good extinction ratio and are effective in suppressing pile up of holes. As the output waveguides 309 to 312 and 314, ridge waveguides embedded in benzocyclobutene (BCB), which is low in permittivity, are used in order to ensure radio frequency bands. As the MMI optical multiplexer 313, a high-mesa type waveguide, which enables strong light confinement and small radiation loss, is used.
The size of the semiconductor chip 322 is 2,000×2,600 μm, the cavity length of the four DFB laser parts 301 to 304 is 400 μm, the waveguide length between the DFB laser parts 301 to 304 and the EA modulator parts 305 to 308 is 50 μm, and the element length of the EA optical modulator parts 305 to 308 is 150 μm.
The optical transmitter module 323 is obtained by mounting the fabricated semiconductor chip 322 in a package having an ultra-small size of 12 mm×20 mm, and is capable of 40 km error-free transmission through a single-mode fiber when operated at 100 Gbit/s at 40° C. These results indicate that the optical transmitter module 323 has sufficient performance as a future-generation 100 GbE transceiver.
Meanwhile, in a case of performing APC in a configuration in which modulated light beams with a plurality of wavelengths are multiplexed as in FIG. 2, it is meaningless to split and monitor part of the modulated light beams between the lenses 316 and 319 as in FIG. 1. Specifically, since a plurality of modulated light beams are multiplexed, detecting a decrease of the modulated light beams does not indicate which DFB laser part a feedback should be given to. For this reason, in a device as illustrated in FIG. 2, the monitoring is performed at the back facet side of each DFB laser part.
FIG. 2 denotes light detectors 1 to 4 for monitoring the DFB laser parts, respectively. The light detectors 1 to 4 monitor the intensities of light beams output rearward from the DFB laser parts. This utilizes the nature of a DFB laser in which it generally outputs a laser light beam rearward at the same time as outputting a laser light beam forward, and the intensity of the light beam traveling forward and the intensity of the light beam traveling rearward are not always equal but are correlated such that as one becomes stronger the other becomes stronger and as one becomes weaker the other becomes weaker. Feedback is applied so as to increase the drive current to a DFB laser part if the light intensity detected by the corresponding light detector drops whereas feedback is applied so as to decrease the drive current if the light intensity rises. In this way, APC is possible.
FIG. 3 illustrates a cross-sectional view of a conventional semiconductor chip in which DFB laser parts, EA modulator parts, and optical multiplexer parts are formed. Reference sign 501 denotes an n electrode, reference sign 502 denotes an n-InP substrate, reference sign 503 denotes an n-InP cladding layer, reference sign 504 denotes an active layer of each DFB laser part, and reference sign 505 denotes a guide layer of the DFB laser part. A diffraction grating is formed in the guide layer 505 by EB (electron beam) lithography. Reference sign 506 denotes a p-InP cladding layer, and reference sign 507 denotes an electrode of the DFB laser part. Further, reference sign 508 denotes an absorption layer of each EA modulator part, reference sign 509 denotes an electrode of the EA modulator part, and reference signs 510 and 511 denote a core layer and a non-doped InP layer of the waveguide (or optical multiplexer), respectively.
At a center portion of each DFB laser part, a ¼ wavelength phase shifter 512 is provided which is obtained by shifting the phase of the diffraction grating by ¼ of the wavelength, to implement a single mode with the oscillation wavelength. In the single semiconductor chip, the active layers 504 of the plurality of DFB laser parts have the same composition, and the pitches of the diffraction gratings are changed to change the respective wavelengths. Also, in the single semiconductor chip, the absorption layers 508 of the plurality of EA modulator parts have the same composition as well.
A light detection layer 513 and an upper cladding layer 514 of a waveguide and an electrode 515 are provided rearward of each DFB laser part to form a light detector. Here, the light detection layer 513 and the upper cladding layer 514 of the waveguide may have the same compositions as the absorption layer 508 of the EA modulator part and the p-InP cladding layer 506, respectively. Also, the light detection layer 513 of the waveguide may have the same compositions as the active layer 504 of the DFB laser part and the guide layer 505 of the DFB laser part.
Meanwhile, while the conventional optical transmitter module illustrated in FIG. 1 and the conventional integrated wavelength multiplexing optical transmitter module illustrated in FIG. 2 are useful, the chirping problem still remains. To solve this, the structure of a semiconductor chip with an SOA integrated EA-DFB laser as illustrated in FIG. 4 has been proposed, in which a semiconductor optical amplifier (SOA) is further integrated with the EA-DFB laser (see Patent Literature 1).
Usually, the length of the SOA part is, for example, about 1/6 of the length of the DFB laser part, and the composition of the SOA part is the same as the composition of the DFB laser part. However, there is no diffraction grating in the SOA part. In the SOA integrated EA-DFB laser, in order to avoid increase in the number of control terminals, the DFB laser part and the SOA part are controlled using the same terminal, that is, electric currents are injected into the DFB laser part and the SOA part in accordance with the resistance ratio between the DFB laser part and the SOA part designed to achieve the desired current distribution.
By using the SOA integrated EA-DFB laser with the structure illustrated in FIG. 4, the chirping problem can be solved. Moreover, the output of the modulated light output can be amplified by the SOA part as well.